1. Field of the Invention
The present invention relates to a water jet propulsion apparatus, and more particularly to a water jet propulsion apparatus for use in a watercraft.
2. Description of the Background Art
A water jet propulsion apparatus of the background art, e.g., corresponding to Japanese Patent Laid-open No. Hei-5-105189, the entirety of which is hereby incorporated by reference, is shown in FIG. 6. The water jet propulsion apparatus has a duct 2 forming a channel in communication with an intake 1a provided on the bottom of a hull 1, an impeller 3 disposed in the duct 2, a bearing 4 of the impeller provided in the duct 2, and a cap 5 closing the rear end of the bearing 4. The duct 2 includes an impeller duct 2a, a stationary vane duct 2b, and a nozzle 2c. The bearing 4 is formed integrally with the stationary vane duct 2b via a stationary vane 2b1.
The impeller 3 is fixed to a drive shaft 6. The front end of the drive shaft 6 is coupled to the output shaft of an engine (not-shown) mounted on the hull 1, and the rear end 6a is rotatably supported by the bearing 4. A stationary vane 5a is formed on the external face of the cap 5, and a stationary vane 2c1 is integrally formed on the internal face of the nozzle 2c. A deflector 7 is rotatably attached to the rear part of the nozzle 2c. 
In such a water jet propulsion apparatus, when the impeller 3 is rotated by the rotation of the engine via the drive shaft 6, water drawn in from the intake 1a is passed through the impeller duct 2a and discharged from a jet port 2c2 of the nozzle 2c to the outside via the deflector 7 to propel the hull 1. The water flow is straightened by the stationary vanes 2b1 and 2c1 in a process of passing the flow through the stationary duct 2b and nozzle 2c. 
In the above-described conventional water jet propulsion apparatus, since the stationary vane 5a is formed on the cap 5, the rotational component of a water stream near the center of the nozzle 2c, e.g., around the cap 5 the rotational component of the water stream can be reduced and the water stream can be straightened to a certain degree. However, it is difficult to form a number of stationary vanes 5a within the limited length of the outer circumference of the cap 5 whose diameter is relatively small.
Therefore, the present inventors have determined that the rotational component of a water stream very close to the external face of the cap 5 cannot be sufficiently reduced with the systems of the background art. Since the stationary vane 5a is formed on the external face of the cap 5, the cap 5, including the stationary vane 5a, is relatively large. In addition, the stationary vane 5a formed on the external face of the cap 5 and the stationary vane 2c1 formed on the internal face of the nozzle 2c have to be repositioned.